Any ophthalmic lens intended to be worn in a frame is associated with a prescription. In ophthalmics, the prescription may comprise a power prescription, either positive or negative, and an astigmatism prescription. These prescriptions correspond to corrections to be provided to the wearer of the lenses in order to correct defects in his vision. A lens is fitted into the frame according to the prescription and the position of the wearer's eyes relative to the frame.
In the simplest cases, the prescription is nothing more than a power prescription. The lens is said to be a unifocal lens and exhibits symmetry of revolution. It is simply fitted into the frame so that the principal viewing direction of the wearer coincides with the axis of symmetry of the lens.
For presbyopic wearers (long-sighted subjects), the value of the power correction in far vision is different from that in near vision, owing to the difficulties of accommodation in near vision. The prescription is therefore made up of a far vision power value and an addition (or power progression) representative of the power increment between the far vision and the near vision; this amounts to a far-vision power prescription and a near-vision power prescription. Lenses suitable for presbyopic wearers are progressive multifocal lenses; these lenses are described for example in FR-A-2 699 294, U.S. Pat. No. 5,270,745 or U.S. Pat. No. 5,272,495, FR-A-2 683 642, FR-A-2 699 294 or FR-A-2 704 327. These progressive multifocal ophthalmic lenses comprise a far vision zone, a near vision zone and an intermediate vision zone, a principal meridian of progression passing through these three zones. They are generally determined by optimization on the basis of a number of constraints imposed on various characteristics of the lens. These lenses are general-purpose lenses in that they are adapted to the different needs of the wearer.
Families of progressive multifocal lenses are defined in which each lens of a family is characterized by an addition, which corresponds to the power variation between the far vision zone and the near vision zone. More precisely, the addition, denoted by A, corresponds to the power variation between a point FV in the far vision zone and a point NV in the near vision zone, which are called the far-vision control point and the near-vision control point, respectively, and which represent the points of intersection of viewing with the surface of the lens for vision at infinity and for reading vision.
In any one family of lenses, the addition varies from one lens to another of the family, between a minimum addition value and a maximum addition value. Usually, the minimum and maximum addition values are 0.75 dioptres and 3.5 dioptres respectively, and the addition varies from 0.25 dioptres in 0.25 dioptre steps from one lens of the family to the other.
Lenses of the same addition differ by the value of the mean sphere at a reference point, called here the base. For example, it is possible to choose to measure the base at the far-vision measurement point FV. Thus, by choosing an addition/base pair, a set of aspherical front faces is defined for progressive multifocal lenses. Usually, five base values and twelve addition values may thus be defined, i.e. sixty front faces. In each of the bases, an optimization for a given power is carried out. This known method makes it possible, starting from semi-finished lenses, only the front face of which is conformed, to prepare lenses suitable for each wearer by simply machining a spherical or toric rear face.
Thus, progressive multifocal lenses usually have an aspherical front face, which is that face of the spectacles on the opposite side from the wearer, and a spherical or toric rear face, turned towards the person wearing the spectacles. This spherical or toric face allows the lens to be adapted to the user's ametropia so that a progressive multifocal lens is generally defined only by its aspherical surface. As is well known, such an aspherical surface is generally defined by the height of all its points. The parameters formed by the minimum and maximum curvatures at each point, or more usually their half-sum and their difference, are also used. This half-sum and this difference, when these are multiplied by a factor (n−1), n being the refractive index of the material of the lens, are called the mean sphere and the cylinder, respectively.
A progressive multifocal lens may thus be defined, at any point on its complex surface, by geometrical characteristics comprising a mean sphere value and a cylinder value, these being given by the following formulae.
As is known, a mean sphere D at any point on a complex surface is defined by the formula:
  D  =                    n        -        1            2        ⁢          (                        1                      R            1                          +                  1                      R            2                              )      
where R1 and R2 are the local maximum and minimum radii of curvature, expressed in meters, and n is the index of the constituent material of the lens.
A cylinder C is also defined by the formula:
  C  =            (              n        -        1            )        ⁢                                                1                          R              1                                -                      1                          R              2                                                  .      
The characteristics of the complex face of the lens may be expressed by means of the mean sphere and the cylinder.
Moreover, a progressive multifocal lens may also be defined by optical characteristics, taking into consideration the situation of the person wearing the lenses. This is because the optical ray-tracing laws result in optical defects when the rays move away from the central axis of any lens. These known defects, which include amongst others a power defect and an astigmatism defect, can generically be called ray obliquity defects.
Ray obliquity defects have already been well identified in the prior art and improvements have been proposed. For example, document WO-A-98/12590 describes a method of determining, by optimization, a set of progressive multifocal ophthalmic lenses. That document proposes to define the set of lenses by considering the optical characteristics of the lenses, and especially the wearer power and the oblique astigmatism under wearing conditions of the lenses. The lens is optimized by ray tracing on the basis of an ergorama associating, with each viewing direction under the wearing conditions, a target object point.